1. Technical Field
The present invention is related to a ball screw device, in particular, to an external circulation type ball screw device.
2. Description of Related Art
Ball screw device plays a significant part role in the field of linear transmissions, in which the configuration of the rolling balls can reduce the friction between the screw shaft and the nut such that the screw shaft and the nut can perform relative movements with each other rapidly and precisely.
As new mechanical structures are designed and built to be smaller in size, the volumes of the ball screws need to be confined in sizes as well in order to fit into such smaller spaces. Among a great number of ball screws, the external circulation type of ball screw is particularly suited to situations where limited spaces are available. As shown in FIGS. 1 to 3, a external circulation type ball screw device comprises a screw shaft 1, having an outer periphery provided with a first screw groove 2, a nut 3, having a through-hole 4 and an inner periphery thereof provided with a second screw groove 5 opposing said first screw groove 2 of said screw shaft 1. And a rolling path form between said first screw groove 2 and said second screw groove 5. A plurality of rolling members 6 are confined to move within the rolling path. As the screw shaft 1 (or the nut 3) is rotated, the rolling members 6 are rolled to drive the nut 3 (or the screw shaft 1) to move axially. Furthermore, a circulating tube 7 inserted into the nut 3 comprises two ends connected to the first screw groove 2 and the second screw groove 5 respectively. After the rolling members 6 move along the bearing path and enter into the circulating tube 7, they would return back to the rolling path again, forming an endless circulation. It shall be noted that during the operation of the ball screw device, the rolling members are not passing through the entire second screw groove, and for the portion of the second screw groove where the rolling members pass through is called an effective thread zone (E.T.Z.). During the operation of the ball screw device, the effective thread zone is required to bear the force exerted by the rolling members. On the other hand, for the portion of the second screw groove where no rolling members pass through is called an invalid thread zone (I.T.Z) which bears no forces exerted by the rolling members.
However, as shown in FIG. 2 and in the abovementioned external circulation type ball screw device, the circulating tube 7 penetrates through the nut 3 and is connected to the bearing path such that the tube wall of the circulating tube 7 to occupies the groove wall effective thread zone of the second screw groove 5 in the nut 3, causing the groove wall to be thinner and as a result and making the effective thread zone of the second screw groove 5 vulnerable to groove wall collapsing as well as the problem of broken teeth of the nut.